feat: implementing support for callbacks

[sydney-runkle]

Introduces a callback system for monitoring MCP server events during tool execution.

Callbacks contain context with server name and tool name and are applied to all servers for a given client.

New API:

from langchain_mcp_adapters.client import MultiServerMCPClient
from langchain_mcp_adapters.callbacks import Callbacks

async def on_progress(progress, total, message, context):
    print(f"[{context.server_name}:{context.tool_name}] {progress}/{total}")

client = MultiServerMCPClient(connections={...}, callbacks=Callbacks(on_progress=on_progress))

Callbacks has the potential to be extended quite broadly to support a variety of notification hooks. We simply match signatures from mcp + add a final context arg which is easily extensible as well.

What's Added:

• CallbackContext - provides server/tool context to callbacks
• Callbacks - main configuration class
• LoggingMessageCallback & ProgressCallback - callback protocols w/ context
• Optional callbacks parameter on MultiServerMCPClient

[christian-bromann]

One comment, otherwise LGTM

[christian-bromann]

This is called mcpServers in JS, do we want to align here? Happy to move to connections and deprecated mcpServer (but keep functionality alive)

[sydney-runkle]

going to leave as is for now based on offline disc